My invention relates to an improved multichannel radio telephone system, and particularly to an arrangement or method for causing a station in such a system to lock on a true idle channel and not lock on a phantom idle channel resulting from intermodulation.
In multichannel radio telephone systems having equally spaced radio frequency channels (that may or may not be contiguous), intermodulation is a frequent problem. To a lesser extent, intermodulation is also a problem in systems having unequally spaced channels. Intermodulation is the production of undesired frequencies representing the sums and differences of the respective desired frequencies being produced by two transmitters. For example, if two radio frequency transmitters are respectively transmitting on frequencies F1 and F2, the general expression for intermodulation products would be NF1.+-.MF2, where N and M are integers, and the order of a product is N+M. The intermodulation products of greatest concern are generated by NF1-MF2, where N=M+1. The strongest intermodulation product would be the third order products of 2F1-F2 and 2F2-F1. If F1 is less than F2, this may also be mathematically described by making use of the difference frequency .DELTA. between F1 and F2. In this case, the third order intermodulation products would be F2+.DELTA. and F1-.DELTA., and the fifth order products would be F2+2.DELTA. and F1-2.DELTA.. Pth order products are thus determined by F2+Q.DELTA., and F1-Q.DELTA., where P=2Q+1. When radiated, the strongest intermodulation signals are usually the third order products F2+.DELTA. and F1-.DELTA.. These intermodulation signals can and frequently do cause improper or false operation of the system.
Accordingly, a general object of my invention is to provide a new and improved multichannel radio telephone system in which false operation resulting from intermodulation is substantially reduced.
Another object of my invention is to provide a new and improved arrangement for use in a multichannel radio telephone system that eliminates false operation resulting from intermodulation in such a system.
One multichannel radio telephone system that is extensively used comprises a terminal controlling fixed stations and semi-intelligent mobile stations. The fixed radio stations and terminal and the satellite or mobile radio stations must all be aware at all times which channel is next in line to be used. This is accomplished by the terminal and fixed stations sending a continuous idle tone on the channel that is to be used next. Each of the satellite or mobile stations scans the locally available channels, and locks on the channel with idle tone to await or to initiate a call or message. Such calls or messages may be either land originated or mobile originated. The nature of this sequence of operation, and the fact that the closely spaced (sometimes contiguous) and evenly spaced channels are particularly prone to intermodulation interference create a well known problem: a working channel and an idle channel are mixed to produce a phantom channel on another frequency. This channel has superimposed on it the audio information carried on both channels. Frequently, this other frequency is the same as that of another channel in the radio telephone system. Hence, a mobile station which is searching for an available channel with idle tone may lock on the phantom channel with idle tone. This mobile station will therefore be blocked from access to the communication system until the phantom idle signal disappears, releasing the mobile station to search again for a true idle channel.
Accordingly, another object of my invention is to provide a new and improved arrangement that prevents a mobile unit from locking on a phantom idle channel in a multichannel mobile radio telephone system.
One way that prior art multichannel radio telephone systems have attempted to eliminate the intermodulation problem has been to energize all of the system transmitters with low power at all times, and to switch to high power either when idle tone is transmitted, or when the station is actively engaged in communications, i.e. busy. This arrangement masks or hides the intermodulation-produced phantom channel signals, so that a mobile station scanning for the true idle channel does not receive the phantom idle channel signals. This has worked fairly well in the traditional single cell radio telephone system having a relatively small number of stations and channels. However, several problems present themselves when this arrangement is applied to the larger multicell type radio telephone systems with more than one channel. Some of these problems are:
A. Emergency backup power capacity is wasted when non-communicating transmitters are activated continuously. PA1 B. Intermodulation phantom channels may appear on a channel that is not locally available in a given cell, but that must still be scanned by a mobile station since the channel is available in the system. PA1 C. Depreciation and increased failures of the radio equipment result, since all of the stations in a system must be on at all times.
Accordingly, a general object of my invention is to provide a new and improved arrangement for multichannel radio telephone systems that eliminates many or all of the problems resulting from the prior art solutions mentioned above.